This research proposal concerns the enzymes which are involved in the activation of bile acids with coenzyme A and their subsequent conjugation with glycine or taurine. The enzymes are (for cholic acid): cholate: CoA ligase (E-I) and cholyl-CoA:glycine/taurine N-acyltransferase (E-II). E-I is a microsomal enzyme which we propose to purify for the first time. We will then characterize its chemical and physical properties and the nature of its attachment to the membrane. We will do a complete study of its catalytic properties, including studies on the properties of the inhibition by conjugated bile acids. E-II has been isolated from the soluble fraction of bovine liver by our laboratory. We propose to characterize its catalytic properties. We will then purify E-II from a non-mammal, characterize its chemical, physical and catalytic properties, and then compare them to those for the mammalian enzyme in order to better understand the evolutionary change that occurred in the enzyme. With this same intention, we will also study the catalytic properties of E-II in some additional mammals and non-mammals and also in a marsupial which is on the border between the two. The developmental pattern of E-II will also be studied in certain key animals. The proposal that contraceptive steroids interfere with bile acid conjugation will be examined by studying the effect of treating rats with ethinyl estradiol on the activities of E-I and E-II.